1. Field of the Invention
The present invention relates generally to wellbore resistivity measurements. More particularly, the present invention relates to correcting erroneous downhole resistivity measurements.
2. Description of the Related Art
Modem petroleum drilling and production operations demand a great quantity of information relating to parameters and conditions down hole. Such information typically includes characteristics of the earth formations traversed by the wellbore, in addition to data relating to the size and configuration of the bore hole itself. The collection of information relating to conditions down hole commonly is referred to as "logging." Logging has been known in the industry for many years as a technique for providing information regarding the particular earth formation being drilled and can be performed by several methods. In conventional oil well wire line logging, a probe or "sonde" is lowered into the borehole after some or all of the well has been drilled, and is used to determine certain characteristics of the formations traversed by the borehole. A wireline sonde may include a source device for transmitting energy into the formation, and one or more receivers for detecting the energy reflected from the formation. The sonde typically is constructed as a hermetically sealed cylinder for housing the sensors, which hangs at the end of a long cable or "wireline." The cable or wireline provides mechanical support to the sonde and also provides an electrical connection between the sensors and associated instrumentation within the sonde, and electrical equipment located at the surface of the well. Normally, the cable supplies operating power to the sonde and is used as an electrical conductor to transmit information signals from the sonde to the surface and to control signals from the surface to the sonde. In accordance with conventional techniques, various parameters of the earth's formations are measured and correlated with the position of the sonde in the borehole, as the sonde is pulled uphole.
One concern for every downhole tool is the accuracy of its measurements. For example, in the prior art, real world constraints have limited the accuracy, and hence the reliability, of downhole resistivity tools. Referring now to FIG. 1, a wellbore 100 in formation 105 surrounds downhole current supply electrodes on resistivity tool 110. Formation 105 may contain high resistivity portion 150 and low resistivity portion 155. Also shown are return B-electrode 120, reference N-electrode 125, and comparator 130. Tool 110 provides electrical current 140 to formation 105. Current 140 flows to return B-electrode 120. Comparator 130, attached to tool 110 and N-electrode 125, measures the potential drop between the tool 110 and the N-electrode 125. The resistivity of the formation 105 may then be calculated based upon this measured voltage differential at comparator 130.
Nonetheless, a calculated resistivity based upon the assembly of FIG. 1 may be inaccurate, particularly when it occupies a formation with low and high resistivity strata. More particularly, measurements between resistivity tool 110 and the N-electrode 125 should ideally approximate the measurements between a resistivity tool 110 and infinity. However, when the N-electrode 125 and B-electrode 120 are spaced relatively near to one another, they interact and affect the voltage measurement at logging device 110. This interaction is particularly pronounced when the formation 150 surrounding the N-electrode 125 and B-electrode 120 has a high resistivity, whereas the formation 155 surrounding logging tool 110 has a low resistivity. The problem under these conditions is that the measured survey voltage from the tool is relatively low. However, there is a very high potential drop to the B-electrode from infinity due to its location in a high resistivity bed. Because the N-electrode is also surrounded by the high resistivity bed, the potential at the N-electrode approaches the potential at the B-electrode and thus a highly erroneous tool reading results. This effect often occurs in the Delaware basin in West Texas and as such is known as the Delaware effect. A similar phenomenon is called the Groningen effect so named after the Groningen formation in Holland.
One attempt to solve this problem in the prior art involved placing the B-electrode 120 at the surface (not shown). By placing the B-electrode 120 at the surface, it was thought that resistivity measurement problems would be solved because the B-electrode 120 would not be proximate to the very high resistive bed surrounding the N-electrode 125. However, this solution was not as effective as had been hoped, with substantial measurement error still present. Schlumberger attempted to correct these errors in their ARI-type laterolog tools. Such corrections are complicated, and are based on mathematical modeling. The correction factors often are dependent upon knowledge that is not known "a priori." For example, the bore hole diameter, and formation and mud resistivity upon the tool (which also must be measured as they are not known beforehand). Further, a system placing the B-electrode at the surface is complicated because placement of the B-electrode 120 on the surface requires control of the current supply at the surface.
An alternate and more successful approach to solving the Delaware effect problem was placement of the N-electrode 125 on the surface in a mud pit or some other location that gives a good electrical connection to the ground. This approach also separates the B-electrode from the N-electrode, and thus was expected to improve the downhole measurements of resistivity. It was also thought that such an approach would not require much of the complexity involved when placing the B-electrode on the surface. While this solution yields improved results over placing the B-electrode 120 on the surface, it still has certain drawbacks. These problems arise from the conductive cable armor that extends downhole and that supports and connects the down hole resistivity tool 110 and B-electrode 120 to the surface. Thus, interaction still occurs between the B-electrode 120 and N-electrode 125.
These and other problems exist in the prior art, and thus there is a need for a device or method to solve these problems.